Revert "protect avbtool logic from very small images"

This should have been landed in AOSP instead of internal master and
it's now causing merger failures. Also, this CL in AOSP

 https://android-review.googlesource.com/#/c/platform/external/avb/+/525522/

will handle the problem this CL is trying to address... so let's merge
that instead after reverting this.

This reverts commit d8f0dbf3.

Test: All unit tests pass.
Change-Id: I9d62740634237a76a68423733b6389f465729e9e